This invention relates to a dielectric body formed of a sintered ceramic material composed of oxides of magnesium, calcium, titanium and aluminum. More particularly, this invention relates to such sintered composite dielectric material formed from a particular composition and made by a process that produces a dense body composed of a multiplicity of titanate and aluminate compounds that cooperate to provide an improved combination of dielectric properties including a dielectric constant within a particularly advantageous range and a low thermal coefficient.
In a radio or the like, a bandpass filter is employed to select a signal within a narrow frequency range, referred to as the resonant frequency, from a broad spectrum. Such filters are based upon a block of dielectric material, most commonly a ceramic material. The resonant frequency is determined by the dielectric properties of the material and by the dimensions of the block. Several ceramic materials are available for manufacturing bandpass filters, including several based upon titanate compounds. In general, a material is required having a low dielectric loss, as indicated by a high Q factor, to minimize energy absorption by the dielectric material that would otherwise reduce resonant signal intensity. In addition, common materials have a dielectric constant greater than 30. A high dielectric constant is desirable to reduce filter dimensions. However, for resonant frequencies above about 2 GHz, the small dimensions necessitated by the short wavelength becomes difficult to reliably obtain with high precision by conventional machining operations. Thus, there exists a need for a material having a dielectric constant less than 25 to produce a high frequency filter having dimensions that are more conducive for manufacture by conventional operations.
Furthermore, under typical operating conditions, a bandpass filter is inevitably subjected to variations in temperature. As temperature increases, filter dimensions are altered by thermal expansion of the material, which tends to shift the resonant frequency. Also, dielectric properties are affected, which also tends to shift the resonant frequency. The combination of these effects is indicated by the effective temperature coefficient at resonant frequency, T.sub.f. A T.sub.f approaching 0 is preferred to minimize the shift in resonant frequency due to variations in the operating temperature.